1. Field of the Invention
The present invention relates, in general, to wireless communications systems, particularly, to a system and method for improved mobile station location, and, more particularly, to a system and method for facilitating the location positioning of a mobile station in a code division multiple access cellular system.
2. Background and Objects of the Present Invention
The evolution of wireless communication over the past century, since Guglielmo Marconi""s 1897 demonstration of radio""s ability to provide continuous contact with ships sailing the English Channel, has been remarkable. Since Marconi""s discovery, new wireline and wireless communication methods, services and standards have been adopted by people throughout the world. This evolution has been accelerating, particularly over the last ten years, during which the mobile radio communications industry has grown by orders of magnitude, fueled by numerous technological advances that have made portable radio equipment smaller, cheaper and more reliable. The exponential growth of mobile telephony will continue to rise in the coming decades as well, as this wireless network interacts with and eventually overtakes the existing wireline networks.
Pursuant to a recent Federal Communications Commission (FCC) Ruling and Order, cellular phone service providers within the United States must provide by October 2001 the capability to locate the position of a cellular or mobile phone making an emergency (911) call within the provider""s system to within 125 meters with about 67% probability, i.e., within one standard statistical deviation. A variety of techniques are currently under study to implement geolocation technology into existing and proposed systems.
Of course, one simple way to get a rough indication of a mobile terminal or station""s (MS) position is to determine the identity of the current cell. A more accurate, but still approximate, mechanism is based upon handovers (including soft handovers), by which the propagation time between the MS and an associated base transceiver station (BTS) is measured. The handover method is simple to implement since it involves very little change in the radio part. Further, the various BTSs in such a system do not need an absolute time reference. The handover technique is generally considered unsatisfactory, however, since a handover to two other geographically located BTSs (needed for trilateration) is rare, particularly if the telecommunications system employs 1-cell frequency reuse, as is understood in the art.
An antenna array solution has been proposed by which an MSxe2x80x2 geolocation may be calculated from an estimated direction and the roundtrip delay of the communication signals. Naturally, the inclusion of a Global Positioning System (GPS) device within the MS would solve the geolocation problem, at a cost, however, of excessive computational and receiver complexity in the MS.
Another proposed solution to the above problem is set forth in Applicant""s co-pending and commonly owned patent application entitled xe2x80x9cSystem and Method for Positioning a Mobile Station in a CDMA Cellular Systemxe2x80x9d, U.S. Ser. No. 08/951,345, filed Oct. 16, 1997, of which the instant invention is a Continuation-in-a-Part, and which is incorporated herein by reference. In the co-pending application, a modified downlink measurement solution is proposed in which signals by the BTSs and MSs are repeatedly idled in order to improve the reception (or xe2x80x9chearabilityxe2x80x9d) of more remote systems, thereby enabling the exchange of timing information and trilateration therefrom.
For Code Division Multiple Access (CDMA) systems, the downlink measurements are made by the MS on signals transmitted by the BTSs, e.g., within the pilot channel data. These methods, however, require an absolute time reference in (or synchronization of) the BTSs. The downlink method of the aforementioned Continuation-in-Part application obtains the requisite timing references through signal cessations.
Other proposed solutions include uplink measurements made by the BTSs based upon MS signals, e.g., a long, known training sequence. Such methods, however, as with the downlink techniques, require an absolute time reference in the respective BTS(s) or that inter-BTS timing is known. A combined uplink/downlink solution by Applicants"" Assignee, which overcomes the timing/synchronization requirement, is set forth in U.S. patent application Ser. No. 08/935,421, entitled xe2x80x9cMethod and System for Determining Position of Cellular Mobile Terminalxe2x80x9d, filed Sep. 23, 1997, which is also incorporated herein by reference. The combined solution set forth in said application involves the utilization of uplink and downlink signal propagation air-times to determine the distance between a mobile station and a base station, thereby avoiding the requirement of the absolute time reference or synchronization.
Finally, power measurement geolocation techniques have been employed which estimate signal path loss and, therefore, distance. Such techniques, however, are not very accurate.
Of the above solutions, only the downlink, uplink and combined downlink/uplink techniques are remotely feasible, but each has fundamental problems. The stand-alone downlink solution, for example, has the inherent problem of hearing a sufficient number of BTSs. This is the so-called xe2x80x9cnear-farxe2x80x9d problem, which is particularly serious in CDMA systems. The combined technique also has some drawbacks in that the positioning may take a considerable time since both downlink and uplink measurements are required. Also, reliability in the combined technique is poorer since hearability is limited to that link (up or down) having the worst performance. Finally, more information bandwidth is required in the combined technique.
With regard to the stand-alone uplink technique, it, too, has some difficulties. For example, the near-far problem in such systems may be combated by increasing the transmitter power of the MS and by transmitting a known signal for a long period of time, all of which has serious effects upon system performance. Transmitting a known sequence requires either overriding speech on the speech channel, potentially causing speech disruption, or sending the pattern in parallel, resulting in increased MS complexity, battery drainage and increased information bandwidth usage. The aforementioned problems worsen if the signal is transmitted over a longer period of time.
As discussed, the above uplink and downlink technique require the known relative timing between the BTSs. Although this problem is immediately solvable by inclusion of a GPS receiver in each BTS, this costly alternative is not feasible for small base station systems (BSS), particularly micro and pico BSSs. Furthermore, reliance upon an independent system means the operator has no control over system performance.
A still further problem, related particularly to a current Wideband CDMA standard (WCDMA), is that although the MS can find the primary and secondary synchronization codes from neighboring BTSs, the particular identities of those transmitting BTSs are unknown, which would mean that any timing information obtained is useless. It is, however, possible to ascertain the correct BTS for each signal by detecting the Broadcast Control Channels (BCCH) of the respective BTSs. This, however, requires that the signal to noise (S/I) ratio be fairly high so as to better capture signals from the more remote BTSs.
By way of background, the basics of a Time Division Multiple Access system and a CDMA system will now be discussed to better illustrate some of the aforementioned problems in geolocation in these two systems, as also set forth in Applicant""s co-pending parent application. For example, in Time Division Multiple Access (TDMA) telecommunications systems a mobile terminal or station (MS) is in communication with a given base transceiver station or system (BTS) during only one of typically eight sequential and repeating timeslots. Other MSs communicate separately with the BTS during the other timeslots. The MS may therefore utilize one or more of the other, unused (by that mobile station) timeslots for other uses, such as positioning. In this manner, the timeslot and frame structure of TDMA protocols may be exploited.
With reference to FIG. 1, there is shown a portion of a cellular telecommunications system having a mobile station 10 in communication with a first BTS 12 and, of course, in communication with another user linked thereto, such as via a Public Switched Telephone Network (PSTN) 14. Additional mobile stations 10A, 10B and 10C, also in communication with the BTS 12, are also shown.
As is understood in the telecommunications art, the MS 10 monitors the strength of its signal link with the BTS 12 and maintains that link until a better signal link occurs, e.g., the MS 10 may move away from the BTS 12 toward a neighboring BTS, such as one of BTSs 12A-12D, and hand over control to that BTS. To accomplish such handovers, the MS 10 also monitors the signal strengths of the neighboring BTSs 12A-12D (and any other such systems within range). TDMA systems utilize frequency reuse algorithms to distribute sets of discrete frequencies in a non-repeating manner so that contiguous communications areas or cells, covered by respective BTSs, do not share frequencies. In this way, the MS 10 in TDMA systems may readily measure the received power from a given BTS on unused timeslots and using frequencies different from that of a neighboring BTS.
Code Division Multiple Access (CDMA) systems, on the other hand, operate very differently from the aforementioned TDMA systems and provide fewer and less advantageous opportunities to exploit the inherent properties of the standard. CDMA protocols do not achieve their multiple access property by a division of the transmissions of different users in either time or frequency, as in TDMA and in Frequency Division Multiple Access systems, but instead make a division by assigning each user a different code, which is used to transform a user""s signal into a wideband or spread-spectrum signal which is joined with other such signals from other users. As is understood in the art, a receiver receiving multiple wideband signals uses the code assigned to a particular user to transform the wideband signal received from that user within the combined signal back to the original signal. Additionally, and with reference again to FIG. 1, each BTS in a CDMA system uses the same frequencies, thereby further limiting the exploitation of distinguishing features.
Accordingly, under CDMA protocols, particularly the current IS-95 standard, it is a non-trivial task to modify the standard to accomplish the FCC mandate. One particular problem encountered is estimating an MSxe2x80x2 position when the MS is relatively close to a given BTS, e.g., MS 10 and BTS 12 in FIG. 1. In operation, the determination of an MSxe2x80x2 position entails using either several base stations, e.g., BTSs 12 and 12A-12D, measuring the time delay of at least three of their signals to the MS 10, or the MS 10 itself measures the time delays to several of the BTSs. As shown in FIG. 1, if the MS 10 is close to BTS 12 and the neighboring BTSs 12A-12D are doing the positioning measurements, e.g., through signal time delay, then the signal from the MS 10 may be too weak for the much more distant base stations, i.e., BTS 12C, to measure. Conversely, if the MS 10 in this situation were doing the measurements, then the strong transmission power from the adjacent BTS 12 may drown out the signals, on the same frequencies, from all of the more distant BTSs 12A-12D.
There is, therefore, a need to provide an improved system and method for determining the geographical position of a mobile station within a CDMA environment.
It is, accordingly, a first object of the present invention to provide such an improved system and method for mobile station location positioning.
It is also an object of the invention that the system and method of the present invention substantially adhere to CDMA protocols, for example, the IS-95 standard, and the Universal Mobile Telephone System (UMTS) proposal for Wideband CDMA (WCDMA).
It is a further object of the invention that the system and method set forth herein allow telecommunications systems operating in CDMA (and WCDMA) to meet the upcoming FCC requirements for mobile station positioning within the United States and any subsequent countries requiring such geolocation positional accuracy.
It is a still further object of the present invention that the system and method facilitate mobile station positioning in a variety of contexts, including situations where the mobile station is adjacent one base transceiver system and distant from contiguous or neighboring base transceiver systems.
The present invention is directed to a system and method for improving the accuracy of a location estimation measurement of a mobile station within a telecommunications system. Transmissions from surrounding base transceiver systems are received by a reference location device and any inter-base transceiver system timing differences resolved. A location estimate of the mobile station within the telecommunications system is then obtained.